1. Field of the Invention
The present invention relates to a thermal ink-jet ink having excellent storage stability and an ink cartridge for containing the ink.
2. Description of the Related Art
An ink-jet recording method is a recording method involving causing small ink droplets to fly onto plain paper or onto a dedicated glossy medium to form an image. The method has rapidly become widespread in association with the advancement of reduction in price of recording apparatuses and an increase in recording rate of such apparatuses. In particular, there has been a growing need for photographic quality because digital cameras have become widespread, so further improvements in image quality and printing speed have been requested. Accordingly, techniques more sophisticated than the conventional ones have been requested, and examples of the techniques include a reduction in size of ink droplets, an increase in density of nozzle arrangement, an increase in length of a head in association with an increase in number of nozzles, and control of discharge of ink droplets.
On the other hand, a thermal ink-jet recording mode is a mode involving foaming ink by utilizing thermal energy, and discharging the ink to a recording medium. The mode enables high-speed, high-density, high-definition, high-quality recording, and is suitable for colorization and a reduction in size of a recording apparatus. A general head to be used in the recording mode includes a substrate for an ink-jet recording head on which a heating resistor for foaming ink and wiring, electrically connected to the heating resistor, are formed, and a passage for discharging the ink on the substrate.
In addition, the substrate for an ink-jet recording head is modified in various ways in order to save electrical energy input and prevent a reduction in lifetime of the substrate resulting from destruction of a heating portion in association with foaming of ink. In particular, a protective layer for protecting a heating resistor disposed between a pair of wiring patterns from ink is modified in many ways.
It is advantageous for the protective layer to have a high thermal conductivity or a small thickness from the viewpoint of thermal efficiency. However, it is advantageous for the protective layer to have a large thickness from the viewpoint of protection from ink of wiring connected to the heating resistor. Accordingly, the thickness of the protective layer must be set to an optimum thickness from the viewpoints of energy efficiency and reliability. In particular, a layer in contact with ink is affected by both cavitation damage due to the foaming of the ink, that is, mechanical damage, and damage due to a chemical reaction with an ink component at a high temperature, that is, chemical damage, so the influences of the mechanical damage and the chemical damage need to be sufficiently taken into consideration.
Therefore, the protective layer of an ink-jet substrate generally includes an upper layer (i.e., a layer in contact with ink) having high stability against mechanical damage and chemical damage, and a lower insulating layer for protecting wiring. Specifically, in general, a Ta layer, which is extremely stable both mechanically and chemically, is formed as the upper layer, and an SiN layer, SiO layer, or SiC layer, which can be easily formed by an existing semiconductor manufacturing apparatus and is stable, is formed as the lower layer.
More specifically, an SiN layer having a thickness in the range of 0.2 μm to 1 μm is formed as a protective layer on wiring, and then an upper protective layer is formed. A Ta layer having a thickness in the range of 0.2 μm to 0.5 μm is formed as the upper layer, which is referred to as a “cavitation-resistant layer” because the layer serves as a layer resistant to cavitation damage. With this constitution, both the lifetime and reliability requirements of a heating resistor of an ink-jet substrate are satisfied.
Japanese Patent Laid-Open No. 05-330048 discloses, as an ink-jet technique using a thermal head, an ink-jet head using a material containing Si, N, or Ir at a specific ratio for a heating resistor in order to improve durability and thermal conversion efficiency. There is also disclosed ink containing a chelating reagent at a specific concentration from the viewpoints of improvement in discharge durability of a thermal head and improvement in suppression of kogation in a heating portion due to discharge (see Japanese Patent Laid-Open No. 06-093218). There is also disclosed ink containing an ammonium salt of an acid having a methyl group or a methylene group, and a carboxyl group (see Japanese Patent Laid-Open No. 2002-012803). These conventional techniques relate to suppression of corrosion of a Ta layer in association with discharge duration or suppression of kogation on the Ta layer when the Ta layer is disposed as a surface layer of the protective layer of a heating portion. In addition, ink contains a specific compound at a specific concentration, whereby a balance between the deposition of kogation and the corrosion of the Ta layer due to discharge duration is optimized for lengthening the lifetime of the ink.
In an ink cartridge in which an ink-jet head is directly mounted on an ink tank so that the head and the ink tank are integrated with each other, in view of cost and production, the head needs to be directly mounted on a plastic casing which constitutes the ink cartridge. Accordingly, the use of such a thermal-mode ink-jet head as described above easily causes heat accumulation due to foaming in association with increases in number of nozzles and in density of nozzle arrangement, thereby decreasing thermal efficiency in some cases. In addition, a constitution having high thermal efficiency is demanded from the viewpoint of saving of electrical energy input.
Therefore, the inventors of the present invention have examined constituting the protective layer using only a lower insulating layer mainly composed of a silicon nitride, a silicon oxide, or a silicon carbide, such as SiN, SiO, or SiC, without using the upper layer in the above-described protective layer including the upper layer and the lower layer. Such a constitution provides advantages in thermal efficiency and is also advantageous, because the layer constitution is simplified, in that it can minimize or avoid an increase in cost as well as minimize or avoid a reduction in yield due to a defect of the layer. Further, the constitution provides advantages in durability against cavitation damage because the thermal efficiency is improved and electrical energy input can therefore be reduced. That is, the constitution of the protective layer using only the insulating layer that has been conventionally a lower layer as described above is suitable for an integrated ink cartridge including a thermal-mode ink-jet head.
However, as a result of further detailed investigation, the inventors of the present invention have found that a thermal-mode ink-jet head with the constitution described above involves the following new problems. That is, the inventors have newly found that an insulating protective layer composed of a silicon nitride, a silicon oxide, or a silicon carbide as a main component dissolves in ink, and thus decreases in thickness during long-term storage not involving foaming, for example, assuming physical distribution. In addition, the reduction in thickness of the protective layer increases the foaming energy that will be applied to the ink, thereby easily causing image deterioration in association with a discharge failure due to abnormal foaming and a reduction in printing durability due to an abnormal increase in temperature of a heating portion when a driving pulse is applied to the head. This is a first problem addressed by the inventors of the present invention.
Without intending to be bound by theory, the inventors consider the above-mentioned phenomenon to occur via a mechanism different from a thermal shock at high temperature and high pressure due to a conventionally known foaming phenomenon and corrosion of a heating portion due to cavitation or the like. Investigation conducted by the inventors of the present invention shows that there is no correlation between the durability of a heating portion for foaming and the dissolution of an insulating protective layer due to the above-mentioned physical distribution storage not involving foaming. Even in a combination of ink and a head each having sufficient discharge durability for foaming, the storage stability of the ink deteriorates owing to the above-mentioned dissolution phenomenon in some cases.
On the other hand, ink may contain impurities of a constituent material of the ink and a polyvalent metal eluted from a member that contacts the ink. The inventors of the present invention have confirmed that the polyvalent metal precipitates at a nozzle portion of an ink-jet head to cause a discharge failure of the ink. In particular, an improvement in image quality is currently advanced, and thus, the number of nozzles and the density of nozzle arrangement are being further increased in an ink-jet head. Particularly in such a head, a discharge failure of ink greatly affects the formation of an image, so the discharge failure of the ink must be prevented as much as possible. The inventors of the present invention have defined the prevention of the discharge failure of the ink as a second concern of the present invention.
Further, the inventors of the present invention have found that ink containing a specific coloring material dissolves the above-mentioned insulating protective layer. A coloring material in ink is selected with the emphasis on its weatherability and color developability because the coloring material is an important factor having a significant influence on the image properties of a printer. Accordingly, a coloring material credited with an ability to improve the quality of an ink-jet recorded image can cause dissolution of the insulating protective layer to reduce the thickness of the protective layer in some cases. In addition, the reduction in thickness of the protective layer increases foaming energy that will be applied to the ink, so image deterioration in association with a discharge failure due to abnormal foaming and a reduction in printing durability due to an abnormal increase in temperature of a heating portion easily occur upon application of a driving pulse to a head. This is a third problem addressed by the inventors of the present invention.